3. It has been over two years since she worked here.
=> The last time she has worked here for over 2 years.
13. The poem is short, but many students can't leam it by heart.
=> Even though The poem is short,many students can't leam it by heart.
14.I bought this air-conditioner because it consumes less electricity than the others.
=> This air-conditioner was bought because it consumes less electricity than the others.
19. The policeman said to Mary, "Could you show me your driving license, please?"
=> The policeman told Mary could I show he my driving license.